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I just saw "$${x%% *}" in a makefile, which means "${x%% *}" in sh. Why it is written in this way ?
https://unix.stackexchange.com/questions/28994/how-can-a-makefile-detect-whether-a-command-is-available-in-the-local-machine
determine_sum = \
sum=; \
for x in sha1sum sha1 shasum 'openssl dgst -sha1'; do \
if type "$${x%% *}" >/dev/null 2>/dev/null; then sum=$$x; break; fi; \
done; \
if [ -z "$$sum" ]; then echo 1>&2 "Unable to find a SHA1 utility"; exit 2; fi
checksums.dat: FORCE
$(determine_sum); \
$$sum *.org
Also, how to search ${x%% *} in Google ?
648
Invoking a Bash shell with the -x option causes each shell command to be printed before it is executed. This is especially useful for diagnosing problems with installation shell scripts.
The operator "%" will try to remove the shortest text matching the pattern, while "%%" tries to do it with the longest text matching.
$1 is the first command-line argument passed to the shell script. Also, know as Positional parameters. For example, $0, $1, $3, $4 and so on. If you run ./script.sh filename1 dir1, then: $0 is the name of the script itself (script.sh)
${0} is the first argument of the script, i.e. the script name or path. If you launch your script as path/to/script.sh , then ${0} will be exactly that string: path/to/script.sh . The %/* part modifies the value of ${0} . It means: take all characters until / followed by a file name.
The ${x%% *} substitutes the value of x variable with the leftmost space and everything to the right of it removed.
See http://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html
To the second question I have no answer.
141
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